Plasma display panels, called hereafter PDPs, are flat-type display screens. There are two large families of PDPs, namely PDPs whose operation is of the DC type and those whose operation is of the AC type. In general, PDPs comprise two insulating tiles (or substrates), each carrying one or more arrays of electrodes and defining between them a space filled with gas. The tiles are joined together so as to define intersections between the electrodes of the said arrays. Each electrode intersection defines an elementary cell to which a gas space corresponds, which gas space is partially bounded by barriers and in which an electrical discharge occurs when the cell is activated. The electrical discharge causes an emission of UV rays in the elementary cell and phosphors deposited on the walls of the cell convert the UV rays into visible light.
In the case of AC-type PDPs, there are two types of cell architecture, one called a matrix architecture and the other called a coplanar architecture. Although these structures are different, the operation of an elementary cell is substantially the same. Each cell may be in the ignited or “on” state or in the extinguished or “off” state. A cell may be maintained in one of these states by sending a succession of pulses, called sustain pulses, throughout the duration over which it is desired to maintain this state. A cell is turned on, or addressed, by sending a larger pulse, usually called an address pulse. A cell is turned off, or erased, by nullifying the charges within the cell using a damped discharge. To obtain various grey levels, use is made of the eye's integration phenomenon by modulating the durations of the on and off states using subfields, or subframes, over the duration of display of an image.
In order to be able to achieve temporal ignition modulation of each elementary cell, two so-called “addressing modes” are mainly used. A first addressing mode, called “addressing while displaying”, consists in addressing each row of cells while sustaining the other rows of cells, the addressing taking place row by row in a shifted manner. A second addressing mode, called “addressing and display separation”, consists in addressing, sustaining and erasing all of the cells of the panel during three separate periods. For more details concerning these two addressing modes, a person skilled in the art may, for example, refer to U.S. Pat. Nos. 5,420,602 and 5,446,344.
Whatever the addressing mode used, there are many problems associated with the temporal integration of the cells operating in on/off mode. One problem, that of contouring, consists of the appearance of a darker or lighter, or even coloured, line upon displacement of a transition area between two colours. The contouring phenomenon is all the more perceptable when the transition takes place between two very similar colours that the eye associates with the same colour. A contour sharpness problem also occurs with moving objects.
FIG. 1 shows a time division for displaying two consecutive images with a transition that moves. The total display time of the image is 16.6 or 20 ms, depending on the country. During the display time, eight subfields associated with periods of weights 1, 2, 4, 8, 16, 32, 64 and 128 are produced so as to allow 256 grey levels per cell. Each subfield makes it possible for an elementary cell to be illuminated or not for an illumination time equal to the weights 1, 2, 4, 8, 16, 32, 64 or 128 multiplied by an elementary time. The illumination times are separated by erasing and addressing operations during which the cells are off.
A transition on one colour between a level 128 and a level 127 is represented for an image I and an image I+1 with a shift of 5 pixels. The integration performed by the eye amounts to temporally integrating the oblique lines shown. The result of the integration is manifested by the appearance of a grey level equal to zero at the moment of the transition between the levels 128 and 127, whereas the human eye does not make a distinction between these two levels. When the transition occurs from the level 127 to the level 128, a level 0 appears, conversely, when transition occurs from the level 128 to the level 127, a level 255 appears. When the three primary colours (red, green and blue) are combined together, this change in level may be coloured and become even more visible.
A first solution consists in “breaking” the high weights in order to minimize the error. FIG. 2 shows the same transition as FIG. 1 using seven subfields of weight 32 instead of three subfields of weights 32, 64 and 128. The eye's integration error then occurs on a maximum value equal to a level 32. Many other solutions have been provided, by varying the weights of the subfields so as to minimize the error. However, whatever the solution adopted for the brightness distribution of the various subfields, there always remains a display error due to the coding.
In European Application No. 0 978 817 (hereafter called D1), it is proposed to correct the image according to the observed movements. In D1, movement vectors are calculated for all the pixels of an image to be displayed and then the subfields are moved along these vectors according to the various weights of the subfields. The correction thus obtained is shown in FIG. 3. The result of this correction gives an excellent result on the transitions that cause contouring effects, as generally the areas belonging to a transition subject to contouring move with the same movement vector.
However, the correction described in D1 has a few drawbacks when put into practice on sequences in which the objects cross over. FIG. 4 illustrates a movement vector field obtained from estimators of the prior art. Associated with each point of the current image (image I) is a movement vector indicating the direction of the movement with respect to the previous image (image I−1). When a moving object moves in front of a background, part of the background appears while another part of the background disappears. If it is attempted to displace the subfields of the current image along the movement vectors, a conflict area 1 and a hole area 2 appear. The conflict area 2 is characterized by the crossing of the movement vector, which imposes two values on a given subfield for a given point. The hole area is characterized by the absence of information.